Academic journal article Proceedings of the American Philosophical Society

Exploring Mars for Evidence of Habitable Environments and Life1

Academic journal article Proceedings of the American Philosophical Society

Exploring Mars for Evidence of Habitable Environments and Life1

Article excerpt

A CENTRAL GOAL of Mars exploration is to establish whether life has ever existed there and, whether it did or not, the extent L to which the Mars environment could have sustained life. The evidence that Earth and Mars might have maintained similar environments during their early histories, together with evidence that life emerged early in Earth history, has heightened support for the possibility that life also existed on Mars. Although the search for evidence might ultimately require dedicated life-detection missions, an array of precursor orbiters and landers is required to develop a detailed and global perspective on where and how to conduct those dedicated missions.

Accordingly, searching for evidence of life has become a unifying theme for characterizing Mars as a planetary system. Habitable conditions and the emergence and evolution of life are intimately linked to evolving planetary environments. Understanding how geological, geophysical, and atmospheric processes have interacted and changed is an essential aspect of characterizing those environments. Those sites most likely to have sustained life and to have retained evidence of its presence are the most promising places to search for "biosignatures," which are features that are created only by life and can persist long after their formation.

This review focuses on recent exploration for evidence of past Martian environments that might have sustained life. First, an effort is made to define the concept of a habitable environment. This definition requires a working concept of life as well as a statement of its essential requirements. Second, key global-scale observations of Mars are summarized. They offer insights as to whether any past environments might have been habitable. Third, observations by NASA's Mars Exploration Rover mission are reviewed; these provide the most detailed studies so far of sites that might have been habitable. Finally, the current state of the search for life on Mars is summarized.

WHAT IS LIFE?

Searching for additional examples of life beyond Earth promises to deepen our understanding of life's most fundamental characteristics and its broadest diversity. Exotic habitable environments conceivably have sustained exotic examples of life. Accordingly, we must be prepared to navigate a "halo zone" that extends beyond the characteristics or limits of life in our own biosphere. Still, the design and implementation of strategies to search for life require a working model of what should be sought in the near term. A working model based on our example of fife on Earth seems justified for at least two reasons. First, as stated above, the Martian environment has been more Earth-like than that of any other planet in our solar system. Accordingly, any life on Mars might resemble life on Earth in fundamental ways. Second, it should be understood that this is a "first draft" model that will necessarily be edited in response to ongoing research and exploration.

The NRC Committee on the Limits of Organic Life (Baross 2007) identified the following key traits that are probably shared by all life:

* It must exploit thermodynamic disequilibrium in the environment in order to perpetuate its own state of disequilibrium.

* It is probably chemical in essence, and most probably consists of interacting sets of molecules having covalently bonded carbon atoms, including a diversity of "heteroatoms" (such as N, O, P, etc., in terrestrial organisms) that promote chemical reactivity.

* It probably requires a liquid solvent to support such molecular interactions.

* It probably employs a molecular system capable of Darwinian evolution.

A useful model should go further by stating the apparent requirements, composition, and basic functions of life. These requirements form the basis for inferring the kinds of environments to be explored and observations to be made during missions. The NRC Committee on an Astrobiology Strategy for the Exploration of Mars (Jakosky 2007) proposed that any Martian life forms might exhibit the following characteristics:

* They are based on carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, and the bio-essential metals of terrestrial life. …

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